The present disclosure relates generally to a method of increasing the network density or reducing the porosity of polyacrylonitrile fiber. More particularly, the present disclosure relates to carbon fibers having improved tensile strength and tensile modulus.
Carbon fibers have been used in a wide variety of applications because of their desirable properties, such as high strength and stiffness, high chemical resistance and low thermal expansion. For example, carbon fibers can be formed into a structural part that combines high strength and high stiffness, while having a weight that is significantly lighter than a metal component of equivalent properties. Increasingly, carbon fibers are being used as structural components in composite materials for aerospace applications. In particular, composite materials have been developed wherein carbon fibers serve as a reinforcing material in a resin or ceramic matrix.
In order to meet the rigorous demands of the aerospace and auto industries, it is necessary to continually develop new carbon fibers having both high tensile strength (about 1,000 ksi or greater) and high modulus of elasticity (about 50 Msi or greater), as well as having no surface flaws or internal defects. Carbon fibers having individually higher tensile strength and modulus can be used in fewer quantities than lower strength carbon fibers and still achieve the same total strength for a given carbon fiber-reinforced composite part. As a result, the composite part containing the carbon fibers weighs less. A decrease in structural weight is important to the aerospace and auto industries because it increases the fuel efficiency and/or the load carrying capacity of the aircraft or auto incorporating such a composite part.
Carbon fiber from acrylonitrile is generally produced by six manufacturing steps or stages. Acrylonitrile monomer is first polymerized by mixing it with another co-monomer (e.g., methyl acrylate or methyl methacrylate) and reacting the mixture with a catalyst in a conventional suspension or solution polymerization process to form polyacrylonitrile (PAN) polymer solution (spin “dope”). PAN, containing 68% carbon, is currently the most widely used precursor for carbon fibers.
Once polymerized, the PAN dope is spun into precursor (acrylic) fibers using one of several different methods. In one method (dry spinning), the heated dope is pumped (filtered) through tiny holes of a spinnerette into a tower or chamber of heated inert gas where the solvent evaporates, leaving a solid fiber.
In another method (wet spinning), the heated polymer solution (“spinning dope”) is pumped through tiny holes of a spinnerette into a coagulation bath where the spinning dope coagulates and solidifies into fibers. Wet spinning can be further divided into one of the minor processes of wet-jet spinning, wherein the spinnerette is submerged in the coagulation bath; air gap or dry jet spinning, wherein the polymer jets exit the spinnerette and pass through a small air gap (typically 2-10 mm) prior to contacting the coagulation bath; and gel spinning, wherein the dope is thermally induced to phase change from a fluid solution to a gel network. In both dry and wet spinning methods, the fiber is subsequently washed and stretched through a series of one or more baths.
After spinning and stretching the precursor fibers and before they are carbonized, the fibers need to be chemically altered to convert their linear molecular arrangement to a more thermally stable molecular ladder structure. This is accomplished by heating the fibers in air to about 390-590° F. (about 200-300° C.) for about 30-120 minutes. This causes the fibers to pick up oxygen molecules from the air and rearrange their atomic bonding pattern. Oxygenation or stabilization can occur by a variety of processes, such as drawing the fibers through a series of heated chambers or passing the fibers over hot rollers.
After oxygenation, the stabilized precursor fibers are heated to a temperature of about 1800-5500° F. (about 1000-3000° C.) for several minutes in one or two furnaces filled with a gas mixture free of oxygen. As the fibers are heated, they begin to lose their non-carbon atoms in the form of various gases such as water vapor, hydrogen cyanide, ammonia, carbon monoxide, carbon dioxide, hydrogen and nitrogen. As the non-carbon atoms are expelled, the remaining carbon atoms form tightly bonded carbon crystals that are aligned parallel to the long axis of the fiber.
The resultant carbon fibers have a surface that does not bond well with the epoxies and other materials used in composite materials. To give the fibers better bonding properties, their surface is slightly oxidized. The addition of oxygen atoms to the surface provides better chemical bonding properties and also removes weakly bound crystallites for better mechanical bonding properties.
Once oxidized, the carbon fibers are coated (“sized”) to protect them from damage during winding or weaving. Sizing materials that are applied to the fibers are typically chosen to be compatible with the epoxies used to form composite materials. Typical sizing materials include epoxy, polyester, nylon, urethane and others.
High modulus of carbon fibers comes from the high crystallinity and the high degree of alignment of crystallites in the fiber direction, while the strength of carbon fibers is primarily affected by the defects and crystalline morphologies in fibers. It is believed that increasing heat treatment temperatures to develop a larger and better aligned graphitic structure can improve Young's modulus while removing flaws has the potential to improve fiber strength.
During the spinning process, the acrylic fiber precursor network density can be estimated by making swelling measurements after the coagulation bath and after each washing or drawing bath. The swelling test method involves collecting a wet fiber sample, washing the sample in deionized water, centrifuging the sample to remove surface liquid, and then measuring the weight of the washed and centrifuged sample (Wa). The sample is then dried in an air circulating oven and then re-weighed to measure the dry fiber weight (Wf). The degree of swelling is then calculated using the following formula:Degree of swelling (%)=(Wa−Wf)×(100/Wf)A lower swelling value for a fiber sample typically indicates lower porosity or an increase in fiber network density.
It has been observed that fiber swelling values as measured above do not always decrease as the fiber progresses from the coagulation bath through the washing and drawing baths. In most cases, fiber swelling measurements tend to increase in the first wash/draw bath before they begin decreasing in subsequent baths. This is indicative of a decrease in fiber network density in the first wash/draw bath relative to fiber network density at the coagulation bath exit. This loss in density is a potential defect in the fiber in that it can negatively affect the tensile strength of the final carbon fiber product.
Attempts have been made to densify drawn precursor fibers by keeping the drawing temperatures of the baths as high as possible. Maximum bath temperatures of 80° C. to 100° C., with the number of draw baths being two or greater, have been used. Hotter draw bath temperatures are beneficial for stretching precursor fiber and for accelerating solvent removal but can result in fiber sticking damage. Further, such techniques for achieving densification tend to make the fiber structure too dense resulting in lower oxygen permeability into the fibers during the stabilization stage, resulting in reduced tensile strength.